The present invention relates to snap-type safety relief valves for use on a pressurized system such as pressure vessel or a flow line, especially snap-type safety relief valves having a consistent low blow-down value.
Snap-type safety relief valves have the advantage of responding very quickly to pressure changes in pressurized systems to which they are attached. Snap-type safety relief valves move to a fully open position almost immediately after the pressure within the pressure vessel rises above a user-determined set pressure. This allows excess gas pressure to escape quickly. Then, when sufficient pressure has escaped, snap-type safety relief valves quickly and crisply move back to a closed position. For an example of a snap-type safety relief valve, see U.S. Pat. No. 3,664,362, which is herein incorporated by reference in its entirety.
A xe2x80x9cblow-down valuexe2x80x9d is the percentage difference between the user-determined set pressure and the pressure in the pressure vessel or flow line when the snap-type safety relief valve snaps closed. For example, if a user sets the set pressure at 100 psi, and valve stays open as gas escapes out of the snap-type safety relief valve until the pressure in the pressure vessel or flow line is 70 psi, then this snap-type safety relief valve has a blow-down value of 30%. For an example of a snap-type safety relief valve having a standard blow-down value, see U.S. Pat. No. 4,799,506, which is herein incorporated by reference in its entirety. Low blow-down valves have a blow-down value of about 15% or less, preferably about 10% or less. Low blow-down valves are desirable because they can minimize the amount of gas that is lost from the pressurized system into the atmosphere during venting, thereby addressing environmental concerns.
Existing low blown-down snap-type safety relief valves do, however, have some problems. One problem is that the blow-down value of the valves are affected by built-up downstream back pressures. The term xe2x80x9cbuilt-up downstream back pressuresxe2x80x9d is well understood in the art and documented in the American Petroleum Institute Recommended Practice 520. The length of outlet piping and the number of elbows that are attached to the outlet of the snap-type safety relief valves contributes to built-up downstream back pressures. Generally, the longer the outlet piping and the greater the number of elbows in the outlet piping, the more built-up downstream back pressures are created.
Built-up downstream back pressures affect the blow-down value of typical snap-type safety relief valves. For example, a manufacturer may sell a snap-type safety relief valve with a blow-down value of 10% that is recommended to be used with 10 feet of outlet piping. At an installation site, the installer may disregard the manufacturer""s recommendations and use 20 feet of outlet piping. In such a case, when the valve is in use, it will experience greater built-up downstream back pressures than the manufacturer designed for. The additional built-up downstream back pressures counteract forces that keep the valve open, and may cause the snap-type safety relief valve to close prematurely. If this occurs while the pressurized system still needs to vent, the snap-type safety relief valve would open again. The valve may then open and close in rapid succession, which is a phenomenon known as chatter. Chatter can shorten the life of a snap-type safety relief valve.
Another problem with existing low blow-down snap-type safety relief valves is that they tend to have a sliding-fit piston/sleeve design. See, for example, the snap-type safety relief valve described in U.S. Pat. No. 3,411,530, which is herein incorporated by reference in its entirety. In these designs, when the piston is raised so that gas may escape, a portion of the sleeve may obstruct the flow path as fluid flows through holes in the sleeve. As fluid escapes, foreign particles tend to accumulate between the sliding surfaces, causing additional friction between the sliding-fit parts. This build-up of foreign particles can cause freeze-up of the piston. This can affect the amount of pressure necessary to open the valve, and it can affect the blow-down value of the valve, making the valve""s performance less predictable. Also, contaminated gas can cause the valve to malfunction.
In addition to the above-mentioned problems, low blow-down snap-type safety relief valves can vary in quality in a number of ways. All snap-type safety relief valves each have a flow coefficient, which represents how unobstructed gas flows through the valve when it is fully open. The higher the flow coefficient, the better. Also, different snap-type safety relief valves vary in their ability to maintain their blow-down value, their performance reliability, their durability, their cost to manufacture, and their ease of use.
A snap-type safety relief valve has been discovered that addresses the problems left unsolved by prior valves. Namely, it has been discovered that a snap-type safety relief valve having sonic flow into the body of the valve and out of a secondary orifice on the valve prevents built-up downstream back pressures from affecting the low blow-down value of the valve. This means that the addition of longer outlet piping on the valve, within limits, will not change the low blow-down value of the valve. Thus, the low blow-down value of the valve is consistent. If an excessive length of piping is added, however, then the flow out of the secondary orifice will no longer be sonic, and the blow-down value will be subject to built-up downstream back pressures. Nevertheless, so long as the flow into the body of the valve and out of the secondary orifice of the valve is sonic, built-up downstream back pressures are prevented from counteracting the forces keeping the valve open. The snap-type safety relief valve of the present invention preferably has a high flow co-efficient, and is easy and inexpensive to use and manufacture.
In one aspect of the invention, a snap-type safety relief valve designed for use under specified operating conditions on a pressure vessel or a flow line having a pressurized gas therein is provided. The safety relief valve includes a body attachable to the pressure vessel or flow line, the body comprising a chamber therein and an inlet and an outlet, the inlet comprising an inlet valve seat, the outlet being adapted to attach to outlet piping. The safety relief valve also includes a disk member closable on the inlet valve seat. Further, the safety relief valve includes a mechanism in the body biasing the disk member to rest on the inlet valve seat with a set force such that when the pressure in the pressure vessel exceeds a set pressure resulting from the set force, the disk member is lifted from the inlet valve seat. The safety relief valve also includes a secondary orifice between the body chamber and the outlet, the secondary orifice being sized so that gas flows from the inlet valve seat into the chamber in a sonic flow and so that gas flows from the chamber through the secondary orifice in a sonic flow when the valve opens due to a pressure in the pressure vessel or flow line exceeding the set pressure during testing under the specified operating conditions.
In another aspect of the invention, a combination of outlet piping and a snap-type safety relief valve is provided. The combination includes a safety relief valve body attachable to the pressure vessel or flow line, the body comprising a valve body chamber therein and an inlet and an outlet, the inlet comprising an inlet chamber upstream of an inlet valve seat, the outlet being attached to the outlet piping, a portion of the outlet piping creating an outlet chamber proximal to the outlet. The combination also includes a disk member closable on the inlet valve seat and an adjustment screw acting on a spring in the body biasing the disk member to rest on the inlet valve seat with a set force such that when the pressure in the pressure vessel exceeds the set pressure resulting from the set force, the disk member is lifted from the inlet valve seat. The combination further includes a secondary orifice between the valve body chamber and the outlet, the secondary orifice being sized so that pressure in the valve body chamber is less than about 50% of the pressure in the inlet chamber and so that pressure in the outlet chamber is less than about 50% of the pressure in the valve body chamber, when the valve opens due to a pressure in the pressure vessel or flow line exceeding the set pressure during testing under the specified operating conditions.
In still another aspect of the invention, an improved low blow-down snap-type safety relief valve is provided. The valve has an inlet, a disk member closable on the inlet, a mechanism biasing the disk member on the inlet, a body, and an outlet. The valve prevents built-up downstream back pressures from changing a blow down value. The improvement on the valve comprises a secondary orifice in the body of the safety relief valve sized to permit gas to escape from the body through the secondary orifice in a sonic flow when the valve is opened during testing under operating conditions specified for the use of the valve.
In yet another aspect of the invention, a method of designing a low blow-down snap-type safety relief valve is provided. The method comprises (a) choosing a set of operating conditions under which the valve will ordinarily be run, including a set pressure; (b) providing a trial snap-type safety relief valve having: (i) a body attachable to the pressure vessel, the body comprising a chamber and an inlet and an outlet, the inlet comprising an inlet valve seat, the outlet being adapted to attach to the outlet piping; (ii) a disk member closable on the inlet valve seat; (iii) a mechanism in the body biasing the disk member to rest on the inlet valve seat with a set force such that when the pressure in the pressure vessel exceeds the set pressure resulting from the set force, the disk member is lifted from the inlet valve seat; and (iv) a secondary orifice between the chamber and the outlet, the secondary orifice having a first diameter; and (v) the snap-type safety relief valve having a first pressure gauge attached thereto, the snap-type safety relief valve being mounted on a pressure vessel, the pressure vessel having a second pressure gauge attached thereto; (c) causing the snap-type safety relief valve to snap open by increasing the pressure in the pressure vessel to exceed the set pressure; and (d) comparing a reading from the first pressure gauge to a reading from the second pressure gauge while the valve is open, (i) if the reading of the pressure of the first pressure gauge is from about 35% to about 50% of the pressure of the second pressure gauge, then the design process is complete; else (ii) if the reading of the pressure of the first pressure gauge is not from about 35% to about 50%, adjusting the diameter of the secondary orifice and repeating steps (c)-(d) until the reading of the first pressure gauge is from about 35% to about 50% of the reading of the second pressure gauge.
In still another aspect of the invention, a method of relieving pressure from a pressurized system is provided. In this method, a snap-type safety relief valve is connected in fluid communication with the pressurized system and the valve has (i) a body surrounding a chamber, (ii) an inlet with an inlet valve seat, (iii) a disk member closeable on the inlet valve seat, (iv) a mechanism biasing the disk member closed on the inlet valve seat, (v) an outlet attachable to outlet piping, and (vi) a secondary orifice between the chamber and the outlet. The method comprises exceeding a set pressure established for the valve, thereby lifting the disk member from the inlet valve seat. The method also comprises flowing gas from pressurized system through the inlet into the chamber in a sonic flow. The method also comprises flowing gas from the chamber through the secondary orifice into the outlet in a sonic flow.
The present invention provides the foregoing and other features, and the advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention and do not limit the scope of the invention, which is defined by the appended claims and equivalents thereof.